The Good Mother Lizard

If we are to understand who this good mother lizard was, what she looked like, where she came from, the first thing to be said is that she was a duck-billed dinosaur. (It seems only right, in view of the meaning of Maiasaura, to use the feminine pronoun.) But then that raises more questions than it answers. Who were the duckbills? Where did they come from? Where does Maiasaura fit among the duckbills? Where do the duckbills fit among the dinosaurs?

Perhaps it's best to start with the dinosaurs themselves. The history of the dinosaurs begins a little over 200 million years ago, in the Triassic period, when the planet was one large landmass set in one giant ocean and the land was dominated by large, flesh-eating reptiles.1 Near the seas and rivers the land would have been green and fertile, heavily forested with conifers and cycads, which looked something like palm trees. But in most areas the climate was dry and vegetation would have been less lush, consisting of evergreen bushes, sedges, or trees that could tolerate conditions of low rainfall.

This was the world in which the dinosaurs evolved. In their turn, they became the dominant large land animals. And they retained this dominance for 140 million years, until the mass extinction at the end of the Cretaceous period. The first appearance of the dinosaurs was not, however, as dramatic as their later domination of the planet. Their evolutionary debut was marked by nothing more than the slight turn of a hip.

The traditional language of evolution uses a kind of shorthand. Fish evolved into amphibians and amphibians into reptiles and one group of reptiles into another group of reptiles called dinosaurs. From this or that kind of dinosaur, these others emerged. This is all correct, but without some background knowledge the process sounds like the transformations of a magician—the handkerchief turns into a bouquet of roses and the roses into a dove. The reality is somewhat different.

Changes in genes, in the DNA that carries the instructions for growing any given organism, provide a variety of sizes, shapes and muscles, of innate capacities for speed or acuity of eyesight among organisms in the same species. Some Thoroughbreds run like the wind; some are stumblebums. The better a male horse runs, the higher the stud fee his owners demand when he moves on from racing to reproducing. The fee is for his genes. Of course, with racehorses we select which individual animals get to pass on their genes; we test them at the track, and the fittest, by our standards, get to spend their lives as broodmares and studs. That's artificial selection. Natural selection is somewhat similar—Darwin called it survival of the fittest. But in natural selection fitness is defined not by speed or intelligence, although these may be significant, but by how many viable offspring an organism leaves. If you happen to be a garden slug, for instance, and your coloring makes you less susceptible to murder by the gardener, you may live longer and your offspring may be more numerous. It's no accident that garden slugs are not fire engine red.

These days there is some discussion about the importance of natural selection, whether some evolutionary change is random in nature, and not selected at all, and what the pace of evolution is. Are there rapid bursts alternating with long periods of little change? Or is the progress a steady and slow one? Still, the broad outline of evolutionary change is clear. And a couple of points are important to note before I discuss how the dinosaurs evolved. The transformation of roses into dove is not a good model for evolutionary change for two main reasons. The transformation is too quick, and the roses simply disappear.

The amphibians did not just disappear when the reptiles emerged. Some, to be sure, evolved into reptiles. That is to say, certain lineages of amphibians experienced certain evolutionary changes in skin and breathing apparatus and skeletal construction. At some point, when the resulting creatures were different enough to pursue life on land without needing ever to return to the water, they obviously required a different classification. In contrast to amphibians, reptiles need not live in watery environments (although some do, just as some mammals do).

We could say that the lineages of amphibians that evolved into reptiles disappeared, but really they continued on in different form. Other lineages faced with competition from the new reptiles simply went extinct, with no descendants. This is true disappearance. Still others stayed the distance, evolving and changing in some ways but remaining amphibians. From frogs to salamanders, the world is full of amphibians today—not the big, fierce, carnivorous amphibians that once prowled or perhaps sprawled through the conifers in the time before the heyday of the reptiles, but amphibians nonetheless. This is true for many forms of life. Certainly early single-celled organisms gave rise to all life on earth. But we've still got plenty of single-celled organisms like amoebas that have themselves been slowly evolving to become better adapted to their environment. In evolution the fates of ancestors are various.

To SKETCH THE RISE of the dinosaurs, we can begin with their immediate ancestors. These were the reptiles called thecodonts, most of which were meat-eaters. Some species of thecodonts had developed a new, rapid means of locomotion. They walked and ran on two legs, instead of sprawling along on four like overgrown lizards as had all reptiles and amphibians before them. The dinosaurs went on to improve this form of locomotion. The first dinosaurs, carnivores like their ancestors, were rapid, efficient, two-legged runners. Later, some forms of the dinosaurs developed four-legged or quadrupedal locomotion, though in a more stable and efficient form than other reptiles had achieved. The clearest difference between the first dinosaurs and the last thecodonts lay in the hip socket. Dinosaur sockets are open (there is a visible hole in the bone into which the thighbone fits), and theco-dont sockets are closed.

From the time the first dinosaurs emerged {Staurikosaurus is the earliest known), their history was one of expansion and diversification. Their 140 million years on earth spanned three geologic periods. After the Triassic period came the Jurassic, when some of the favorite dinosaurs of children evolved—the sauropods, the biggest dinosaurs of all, in fact the biggest land animals ever to exist. The most famous of these, Brontosaurus, is known to almost everybody. (It is, however, known by the wrong name. Brontosaurus is now correctly called Apatosaurus.) Other sauropods had a similar look: thick, pillar-like legs, long, almost snakelike necks and equally long tails. The stego-saurs also flourished during this time, as did Allosaurus and other large carnivores that were the precursors in form of Tyrannosaurus rex.

Then came the Cretaceous, which is probably when the dinosaurs reached their greatest diversity. The horned dinosaurs, including the familiar Triceratops, were widespread. Club-tailed dinosaurs called ankylosaurs also thrived. A wide variety of smaller dinosaurs and predatory dinosaurs of every size flourished in North America, Asia, just about all over the world. And it was during the Cretaceous that the dinosaurs of most interest to us, the duckbills, appeared. They themselves then diversified.

The dinosaurs, along with all the other reptiles that are living or have ever lived, are all part of the class Reptilia. Within this class the dinosaurs occupy different orders, genera and species. When fossils of dinosaurs were first found, in the early 1820s, they were something of an anomaly. Scientists were confronted with fossils of three large, terrestrial and (two of them) herbivorous reptiles that did not quite fit with any of the known reptiles, living or extinct. For one thing, no large herbivorous land reptiles were known. Also, all the new animals had in common an open socket where the femur joined the pelvis, and, finally, they all displayed a new look in the way the pelvis was joined to the spine. So in 1841 Richard Owen, the first head of the British Museum of Natural History, took a good hard look at Iguanodon, Megalosaurus and Hylaeosaurus, as the first three of the new reptiles had been named, and created a new class: Dinosauria. The name means, as every schoolchild learns, "terrible lizards."

Biological classification is, however, full of pitfalls, particularly when the evidence for creating a new group is small. It is also a matter of consensus among paleontologists. There is no absolute proof that this or that classification is correct. Scientists like Owen, who wish to change the system of classification, must amass their evidence, publish it, and try to convince other paleontologists. Owen succeeded, momentarily. But he was dealing with odd bones and fragments, not with skeletons or partial skeletons. In the late 1800s paleontologists determined that there were two separate orders of dinosaurs, the Saurischia and the Ornithischia. So the original class, the Dinosauria, fell out of favor.

The difference between the Saurischia and the Ornithischia has primarily to do with the pelvis. Saurischia literally means "lizard-hipped," and Ornithischia "bird-hipped," and you can see the difference in the drawing on page 71. As to evolutionary history, the saurischians

Maiasaura Pelvis
The pelvic structure of a saurischian (Tyrannosaurus rex, for example) differs markedly from that of the "bird-hipped" ornithischian order, to which Maiasaura belongs.

seem to have appeared first, in the late Triassic, about 200 million years ago. Shortly thereafter the first ornithischians appeared, perhaps descended from the saurischians or perhaps coming from some ancestral reptile that was similar to, but not the same as, the ancestor of the saurischians.

Within each of these orders, there are numerous different suborders, families and genera. To take some of the most common and well-known dinosaurs, all the carnivores—from the large and fearsome Tyrannosaurus rex to the small, quick and also fearsome Deino-nychus—are saurischians. So are the sauropods, such as Apatosaurus. Furthermore, it now seems that birds descended directly from the saurischians. Partly for this reason, a variant of the original class Dinosauria has recently come back into favor. In the 1970s, Robert T. Bakker and Peter Galton published a paper arguing that Dinosauria was a legitimate class but that it should include not only both orders of dinosaurs (ornithischians and saurischians) but the birds as well (now alone in the class Aves). Their proposal has since been a lively subject of discussion in paleontology because what it means, in effect, is that there are still living dinosaurs among us—the birds.

As to the ornithischians, they include a number of suborders, among them the stegosaurs (with a ridge of triangular plates along the spine), the ankylosaurs (with armor and clubs on their tails), the ceratopsians (with horns, like Triceratops) and the ornithopods. Within the ornithopods are many families, including the one that interests us: the Hadrosauridae, also known as hadrosaurs or, in the vernacular term we've been using so far in this book, duck-billed dinosaurs. This is the family to which the genus Maiasaura (which includes only one species, Maiasaura peeblesorum) belongs. Duckbills, hadrosaurs and Hadrosauridae are all words for the same animals, a family of dinosaurs with what look like ducks' bills.

The duckbills appeared in the late Cretaceous. They were all herbivores and they all had a two-legged gait, although some may have used their forelegs to help them out occasionally—in rough terrain, for example. They also all had the snouts that resembled ducks' bills. By the time they evolved, the landmass had separated into continents, and the remains of duckbills are found in Europe, Asia, and North and South America. We don't know on which continent they first evolved. In whatever part of the globe they were, they lived on the coastal plains of one sea or another. (As did all the dinosaurs.) We don't know whether they, or any other dinosaurs, also lived in inland areas, because there are no geological formations that preserve inland habitats from the dinosaurs' time. Deposition or sedimentation, which is what makes sedimentary rock and what preserves fossils, did not occur to a large enough extent in the inlands. Consequently, the known habitat of the duckbills was more or less the same across the globe: the lush, green swamps of the low coastal plains or the semiarid upper coastal plains. The duckbills did not live alone, of course. The horned dinosaurs, the club-tailed dinosaurs and many other large predators inhabited the same territories.

In North America the duckbills first appeared around 100 million years ago during the recession of the Colorado Sea, the event that marks the beginning of the Two Medicine formation. Before that, the big bipedal herbivores were dinosaurs called iguanodontids—the ancestors of the duckbills. Once the duckbills appeared, they themselves evolved into a variety of forms. They and the ceratopsians were the dominant herbivores in the late Cretaceous—one walking on two legs, the other on four. They were preyed on by a variety of carno-saurs such as Albertosaurus, an animal that was a slightly smaller foreshadowing of T. rex.

Within the duckbill family, paleontologists have traditionally recognized two subfamilies: the flat-headed Hadrosaurinae and the elaborately crested Lambeosaurinae, known more informally as the hadrosaurines and the lambeosaurines.2 The guess was that the lambeosaurines evolved from the simpler, less ornately decorated

The duckbills are divided into two separate families: the flat-headed hadrosaurs and the crested lambeosaurs. The Kritosaurus skull {right) shows the hadrosaur's characteristic lack of the well-defined crest that is so apparent in the skull of Hypacrosaurus (below), a member of the lambeosaur family. Maiasaura, with just the hint of a crest, belongs to the hadrosaurs.

Maiasaura Skull

hadrosaurines. (Maiasaura, by the way, is of the plainer variety; she has no fancy crest.) However, I've uncovered some evidence suggesting that this classification is no longer valid.3 The flat-heads and the crested duckbills each seem to have evolved from different sorts of iguanodontids, not from the same one. This is called polyphyletic origin, and it requires that they be considered separate families rather than subfamilies. All this means is that the scientific names for these creatures are now the Hadrosauridae and the Lambeosauridae, a change of one consonant. In colloquial terms we can call them the hadrosaurs and the lambeosaurs, and both are still duckbills.

THE FOSSILS OF DUCKBILLS have played a prominent role in the history of paleontology and in my personal history, in the circuitous route that I followed from my home in Montana to New Jersey and back to Montana to Maiasaura'?, lair on the Peebles ranch. In retrospect, I can't imagine that the most important dinosaur fossils I've ever found could have come from any other kind of animal.

One of the very first dinosaur fossils discovered in North America was a duckbill tooth. It was found in Montana, in 1854, in the Judith River formation by paleontologist Ferdinand Hayden, whom the Indians thought to be insane and therefore holy. He found several different teeth, of different dinosaurs, which is why we can't say whether the hadrosaur tooth was actually the first. Hayden was out in the West during the Indian wars and sometimes found himself in the territory of Indians who, for good reason, were less than hospitable to whites. They had a name for him, and I think if you imagine a paleontologist caught unwittingly in some sort of skirmish and trying to get that last hadrosaur tooth pried out of the mudstone before he gets a bullet or an arrow in him, the name acquires its proper resonance: he was called "the man who picks up stones while running."4

The very first relatively complete dinosaur skeleton found any where was also of a hadrosaur. It was found in Haddonfield, New Jersey, in 1858 by William Parker Foulke, on a vacation from Philadelphia. Up until this time, most dinosaur fossils had been fragments and most, except for Hayden's teeth, had been found in Europe. Foulke found the skeleton in a marl pit and brought it to Joseph Leidy at the Philadelphia Academy of Natural Sciences. Leidy, the preeminent American paleontologist at that time, inspected, named and reconstructed the skeleton of the beast and called it Hadrosaurus foulkii.

After this, of course, hadrosaur discoveries burgeoned. Hadro-saur and lambeosaur bones are probably the most common dinosaur fossils found in the American West. And that's how I became involved, if not entangled, with duckbills. I literally followed in the footsteps of Hayden and other paleontologists who plied the North American Cretaceous deposits. And I found what they found—hadrosaur and lambeosaur bones.

There were no great fossil collections in the West when I was in school. Now there is the new $28 million Tyrrell Museum of Palaeontology in Drumheller, Alberta. In the size and scope of its vertebrate paleontology collection, it beggars every other museum in the world. But that's now. The museum wasn't built until 1986. In the late nineteenth and early twentieth centuries just about everything that came out of the ground, in the United States or in the Canadian West, was carried back east. So, although I had read the histories and haunted the same formations in which the early American paleontologists made their great finds, I had never seen the fossils they had collected. I had been to the Far East, to Vietnam by way of Camp Pendleton in California, but I had never been to the East Coast before I got the job at Princeton.

When I arrived in the East and went hopping from one museum to another with Don Baird, looking at the fossil collections, I found more hadrosaurs and lambeosaurs. For instance, Douglass' juveniles from the Bear Paw shale, which got me interested in looking for babies, were almost all duckbills. And there is one other quite important hadrosaur that I found in a museum.

At first, as a newcomer fresh from the territories, I was awed by the collections. But I soon found out that many of them had fallen into disrepair, that some of the great names I had read about had pulled stuff out of the ground without recording much information about it, so that it was now all but useless, and that in some cases the disorder and chaos of the collections were incomprehensible. One of the museums Don and I went to was the Philadelphia Academy of Natural Sciences. Today, it has one of the best and most modern presentations of dinosaurs in the world, but then it was a mess and hopelessly out of date. Nonetheless, I was entranced by it. It was the museum where some of the great early paleontologists had worked. Joseph Leidy, the first great American vertebrate paleontologist, had done his work there, and it was there that he reconstructed Hadrosaurusfoulkii, the first dinosaur skeleton found.

I had already been to see the site where this partially preserved skeleton had been discovered. (It had become a housing development.) And now I wanted to see the reconstructed skeleton itself. But when Don and I asked to see Hadrosaurus foulkii, we were told it was lost. The skeleton wasn't gone. It hadn't been thrown out. It was lost in the collection, which gives you some idea of the collection's sorry condition. Two of the bones from Hadrosaurus foulkii were on display, set in concrete at the base of another dinosaur, but the rest had disappeared into the clutter. In the back rooms, away from the displays, there were bones lying on the floors and stuffed into little glass cabinets one on top of another in a jumble. There was no order that I could see.

I started going to Philadelphia regularly, sometimes with Don but usually by myself, to look for Hadrosaurus foulkii. And I began to find the bones. First I identified them by color—all the bones from New Jersey had the same black color. Then I went over the old records, tracing catalog numbers and trying out bones to fit them to each other.

Eventually I found the thing and put it back together. This was not exactly a field discovery; it was more a rediscovery. But putting that skeleton back together was enormously satisfying. I got the feeling that I myself was part of the history of paleontology, and it still gives me pleasure to see the reconstruction when I go to the Academy today. There are displays of Maiasaura nests there, and videos of me working in the field, an established paleontologist with a dig, a crew, and grants to support them. But there is also Hadrosaurus foulkii, restored by me, the preparator, who was glad to have any job at all in paleontology.

IT WAS ACCIDENT and availability that led me to the hadrosaurs and lambeosaurs, but there were numerous reasons to stick with them. The duckbills are among the most successful of dinosaurs. Numerous species of duckbills thrived during the Cretaceous all over the earth. In the late Cretaceous they were, with the ceratopsians, the dominant herbivores on land. Studies of the duckbills have long been a thriving paleontological business, so that working on them means entering a rich and varied field. And to my mind the hadrosaurs and lambeosaurs are two of the most sophisticated reptiles ever, living or extinct.

I say this because of their teeth. Teeth are very important in the study of fossils. They are hard, and therefore often well preserved, and they may have a lot to say about how the animal lived if you can decipher the clues they offer. The teeth of duckbills are dramatic. All species have some differences, but a typical duckbill jaw has scores of teeth, always being replaced, arranged in several rows on either side of the lower and upper jaw. These are not subtle clues. This kind of grinding apparatus was almost certainly used for processing plant food, just as the carnosaurs' curved, serrated teeth, like small (or sometimes large) steak knives, were used to cut and rend flesh. The

Don Baird Princeton
A hadrosaur jaw reveals a dental magazine well designed for grinding plant food. The teeth, which met on a bias, were replaced as they wore

duckbills' teeth were very well designed for herbivory. They were the rotary mowers of their day.

All this tooth talk may not sound impressive to a mammal. None of us has a dental magazine like the duckbill's, but we do have our share of impressive chewers. Goats and sheep do a respectable job, as do cows, buffalo, giraffes, horses, elephants—there's a long list. Reptiles have no such list. Among reptiles the duckbills are set apart not because they were great chewers, but because they chewed at all. Very few other reptiles, with the notable exception of the ceratopsians and iguanodontids, have ever been able to chew their food. Most reptiles can bite, cut, shear, chop and swallow, but not chew.5

An even more mundane quality that I find very appealing is that the duckbills have manageable skulls. If you want to look at a Tricera-tops skull, and I've done this several times, you practically have to dig a cave underneath it. It's seven feet long and weighs a thousand pounds. And the separate bones of the skull are fused together solidly. Duckbill skulls are small enough to pick up, and they come apart as well.

Originally, all the duckbills were thought to be aquatic dinosaurs, primarily because of their bills. Some of them also have webbed feet, and their broad tails look as if they would have been good for sculling. Then, in 1964, John Ostrom of Yale published a paper arguing that the tail was quite useful for balance, that the hadrosaurs' teeth were well designed for chewing tough terrestrial plants, and that hadrosaur fossils are as often as not found in terrestrial environments where there would not have been that much water.

I agree with him that many duckbills were probably purely terrestrial. But I'm also convinced that at least a few of them were semiaquatic. One big reason is the nature of their skulls. Some duckbills have kinetic skulls. This means that unlike human beings, who have movable lower jaws attached to rigid skulls, these dinosaurs had skulls in which many of the bones moved.6 Birds also have kinetic skulls, all birds. The movable skulls come in particularly handy for ducks that feed on vegetation in the water. They take in plants and water, then press the upper jaw down on the lower one to push the water out through a strainer system and keep the plants in. One particular genus of hadrosaur, Gryposaurus, has a similar setup in its mouth. The edge of its bill is crenulated, or crinkled, making a kind of filter to trap plant material. Gryposaurus lived in a swampy area near the sea. It had webbed front feet, and it had a deep tail that would have worked well for sculling. Maybe Gryposaurus wasn't semiaquatic, but if it wasn't, no dinosaur was.

Whatever they did in the water, on land duckbills probably moved like birds, with their heads bobbing forward and back.7 They did not look like the dinosaurs that have their tails dragging on the floor in the American Museum of Natural History and a lot of other museums. I've said this many times (and so have other paleontologists; it's not my original insight): the tails of most dinosaurs, not only the duckbills but also the sauropods, were held out straight behind them. The duckbills' tails were reinforced by rigid, ossified tendons that we can still see in many fossil skeletons. For the duckbills, the evidence is particularly good. There's a lambeosaur in the American Museum of Natural History with a neck curved almost like a swan's. It is displayed in a case as if it were swimming. But I think that curve in the neck, which is found in other duckbill skeletons preserved in articulated form, would have made sense if the animal had been walking. A swan's neck curves when it swims, but so does a goose's neck when it walks, or a pigeon's, or a chicken's. Watch the way birds seem to bob their heads forward and back when they walk; this redistributes the weight of the body, which is perched like a seesaw on two legs. Bipedal dinosaurs were built the same way. When a duckbill walked, I think it would have had a curved neck and a bobbing, fluid gait.

I don't know what duckbills looked like when they were running, perhaps like huge, leathery ostriches, but I'm absolutely certain they could run. That was one of their major defenses against predators. Other defenses might have been living in herds, a subject for another chapter, and, probably, the ability to deliver quite a solid kick with the hind legs.

I also suspect that the duckbills could make noise. There has been a fair amount of attention paid to the noses and crests of various duckbills. The crests of the lambeosaurs are apparently extended nares. In other words, the lambeosaurs added on, evolutionarily, to their nasal cavities and got, with the extra space, a greater surface area for receiving odors. They also had large spaces that could serve as resonating chambers for noisemaking. David Weishampel at Johns Hopkins University once made some pipes that you could blow into to approximate the noise the lambeosaurs might have made. He did it for a television show, and it was not meant to be highly sophisticated research, but the pipes worked and there is plenty of evidence that the lambeosaur crests would have worked this way, too. The hadrosaurs were probably also able to make noise. There is a hollow passage in the bone of the upper jaw, in a number of hadrosaurs (including Maia-saura), that could have worked something like a flute. The passage may have been covered or partly covered by skin.

IN THIS WORLD OF DUCKBILLS, Maiasaura was a terrestrial, upland dinosaur typical in some ways of the other hadrosaurs. She lived in the middle of the hadrosaur span, around 80 million years ago in the late Cretaceous period. Maiasaura's most distinctive physical trait is the nature of her skull. This was, of course, what made us realize we had a new species. As I said in the last chapter, after we had finished digging out the first nest of babies in August 1978 we took the adult skull that the Brandvolds had found and brought it back to Bob's yard. We removed the cast from the skull, washed off the dirt and tried to figure out what species we had. At first I thought the skull was from a dinosaur called Prosaurolophus, a fairly well-known hadrosaur. Pro-saurolophus lived in the right time and place, and its fossils show a little crest on its skull—not an elaborate one like the lambeosaurs have, just a small, kind of distinguished little fillip on the top of its skull. The skull from the Peebles ranch had just such a crest. When we were washing off the matrix (stone and dirt) from the skull with the hose, at very low pressure, the first thing we saw was that crest. I was predisposed to think that the skull was from a dinosaur that was already known, because one is more likely to find known than unknown dinosaurs. But then, as the work proceeded over the next hour or two (this gives you an idea of the rate at which we let the water dribble from the hose), I began to see from the snout that this was a different creature altogether.

The skull had a distinctive nose, different from all other hadro-saurs with the possible exception of Telmatosaurus transylvanicus. That dinosaur was found around 1900 by one of paleontology's most eccentric figures, Franz Nopcsa, a Transylvanian baron who was a spy, fossil hunter and itinerant European intellectual, and who ended his life by suicide. Nopcsa's Telmatosaurus, like the skull I was holding, had a long muzzle with a small external naris, or nose hole, and a long

expanse of bone between the nans and the orbit. The naris is more or less where you would expect a hole for a nose to be. The orbit is the hole for the eye. All North American hadrosaurs have, in colloquial terms, short snouts and large nose holes. This skull had a long snout and a small nose hole. That was enough to tell us that we had a new species.8

We finally determined that the babies were members of this same species when, during the winter of 1979, we pored through the baby bones and found a lot of fragments of baby skulls. When we compared them to the adult skull, it became clear that both sets of fossils were from the same species of hadrosaur.

The facial region of Maiasaura's skull is, as one would expect, in the shape of a duck's bill. And at the end, just as at the end of a duck's bill, there were during life ramphothecae—horny growths on the upper and lower jaw. If you look carefully at a duck, you'll see that at the end of the bill is something like a pair of horny lips. These are ramphothe-cae, and they are usually a slightly different color from the bill. We know that Maiasaura had ramphothecae because the bottom and top jaws of a skull don't meet when you close them together; something else was attached to those jaws, in life. Furthermore, the ends of the upper and lower jaws are pebbly and rough, with numerous holes that would have served to allow blood flow directly to the ramphothecae.

Maiasaura's crest would have had some kind of skin or cartilage growth attached to it, so that in life this would have been some kind of display structure—Maiasaura's equivalent of the colorful skin on the top of a chicken's or rooster's head. I have no idea what color Maiasaura was in life. Perhaps brown, or green, maybe red. The skin impressions of other duckbills show an unevenness that may have been reflected in spots, or splotches of color. The splotches could, however, have just been different shades of basic brown or green, like the variations in darkness on an alligator's back.

In terms of stature, Maiasaura was like the proverbial Mama

Bear. She was middle size. The average adult size of the maiasaurs we've found is close to 25 feet from nose to tail. (In describing her physical characteristics I will, in a sense, be jumping ahead of myself, making use of the many Maiasaura fossils we were to find on the Peebles ranch throughout our six years of digging.) I can't say that this is as big as Maiasaura got. Although duckbills were remarkably sophisticated reptiles, they were still reptiles. Mammals and birds have limits to their growth, which is evident in the structure of their bones. Reptiles do not stop growing. An alligator just gets bigger and bigger until it dies, although it gets bigger very slowly toward the end. The bones of dinosaurs show that they did not have any set limits to their growth, so the ultimate size of any given dinosaur was subject to diet, time and environment. We have, for instance, two very big maiasaurs that we found on the Peebles ranch, and these would have been at least 30 feet long. Probably maiasaurs didn't often get bigger than that, but I can't say for sure. That's not quite as big as it sounds. Hadrosaurs weren't built like elephants or rhinoceroses. They were more slender, or gracile. For its maximum 30 feet, a maiasaur probably weighed two or three tons, depending on how thin or full-fleshed we imagine it. A powerful draft horse, much shorter in length and height, can weigh a ton. As to how this size fits with other varieties of dinosaur, there were dinosaurs of all shapes and sizes. Some were as small as chickens. One may have been as small as a starling. And others, like the sauropods, were incomprehensibly huge. Even though two-thirds of the 75-foot-long Apatosaurus was neck and tail, it still weighed about 30 tons.

Maiasaura had massive hind legs and somewhat thinner fore-limbs. All the duckbills had hind legs that were bigger and stronger than their forelimbs, but there was considerable variation in the size and heaviness of the bones in both sets of limbs. Some had very heavily built forelimbs, with the humerus (the upper bone) particularly massive. Most of these were lambeosaurs. Maiasaura was constructed in the opposite fashion. Of course, you have to remember we are talking

Maiasaura, depicted in her probable walking posture.

about a 25- to 30-foot dinosaur. A Maiasaura humerus is still big— about two feet long, as thick as a two-inch pipe in spots and in other sections flat and wide. Only for a dinosaur would this be a welterweight humerus. Maiasaura's hind legs were much more massive. I'm tempted to guess that this makes Maiasaura more bipedal than the other hadrosaurs. The ones with the longer, thicker forelimbs might more easily have gone on all fours if the occasion called for it. Maiasaura could not have depended too much on her forelimbs to support her weight, and the hind legs were certainly massive enough to enable her to get around quite well.

Like all other hadrosaurs, she had three toes on her hind feet and four digits on her front feet. Although we have no skin impressions preserved of Maiasaura, I would be willing to bet that, like some other hadrosaurs whose skin impressions have been preserved, she had a frill running down the backbone that served no purpose other than display. It may well have been more prominent on the males. I say this because I know, for reasons I'll explain later, that Maiasaura was a social herd animal, and modern social animals often have characteristics, like antlers, that serve only as displays to attract the attention of potential mates.

The final thing to be said about the physical Maiasaura has to do with her evolution. Maiasaura was a physically conservative, but nonetheless advanced dinosaur.9 In her overall shape, and particularly in the contours of her face, she shows little evolutionary change from her iguanodontid ancestors. In those respects she is something like a generalist, a generalized hadrosaur. In her teeth, however, and in some other characteristics, she has changed quite a bit. Her dental battery is more elaborate than that of her ancestors, and the teeth are different in form from those of the iguanodontids and other early hadrosaurs.

Another kind of animal might show more dramatic, rapid evolutionary change in, say, the shape of its snout. A good example of this kind of animal is an as yet unnamed hadrosaur that comes from the very bottom of the Two Medicine formation. This dinosaur was found by Barnum Brown of the American Museum of Natural History in 1916 near the Two Medicine River, which gave the formation its name. He thought it was some kind of kritosaur, which is a genus of hadrosaur. I don't think he got it right, and I'm working on giving it a new name, but I haven't yet figured out what kind of dinosaur it really is. I do know that the creature is very close to its iguanodontid ancestors; it is one of the first of the hadrosaurs to appear. As might be expected, it has a lot of primitive, iguanodontid-like characteristics, such as its teeth. And yet it also has a greatly extended external naris, or nose hole, in a very noticeable arch. Perhaps for the time being we should call it hook-nose. The nose arch sounds insignificant, but it's a clue. In later deposits in the Judith River formation (which preserve the same time period as that in which Maiasaura lived, but in the lower rather than upper coastal plain) there is at least one whole lineage, with several genera, of hadrosaurs with those nose arches. They look very much like descendants of the original hook-nose.

These two animals, Maiasaura and hook-nose, seemed to me to be involved in two different evolutionary courses: one a rapid radiation of new genera and species, the other a process of slow, gradual refinement. Both courses relate to what was going on with the inland sea, which was the dominant factor in all of North American Cretaceous life. In the first chapter I described the transgressions and regressions, or expansions and contractions, of the Cretaceous sea. Each time the sea expanded, it ate up coastal plain and, in effect, destroyed habitat. Over the course of thousands, or hundreds of thousands, or millions of years, it squeezed all the animals living on those coastal plains into a smaller and smaller area until, in some of the transgressions, it reached to the mountains themselves. Just before the beginning of the Two Medicine formation, such a transgression occurred. The result was that all the species and genera that survived the loss of habitat were pushed into small pockets in the mountains.

Now, this is a recipe for rapid evolution. The work of the renowned evolutionary biologist Ernst Mayr showed that geographic isolation promotes the development of new species because a small group changes, evolutionarily, more quickly than a large population. And according to the recent punctuated equilibrium theory of how evolution occurs, proposed by the paleontologists Niles Eldredge and Stephen Jay Gould, isolation and other stresses promote periods of rapid evolution that punctuate long periods of relative stability during which there is little change. Robert Bakker has applied this theory to the movements of the Cretaceous inland sea to suggest that the transgressions caused numerous extinctions and rapid speciation.

I think Maiasaura and hook-nose provide evidence to support and add to these ideas. They also suggest future avenues of study. What we find in sediments that date from before the transgression of the Colorado Sea are iguanodontids. As the transgression occurred, the growing sea wiped out habitat and squeezed everybody into the mountains. We don't have a good record of what went on during this time, but we do have the Two Medicine formation from the period when the sea began to recede—and we find in it not iguanodontids but hadrosaurs.

Another kind of evolutionary pressure had been introduced when the sea receded. Suddenly vast new territories, new ecological niches, were opened up to be colonized by opportunistic species. With all this open space, another period of rapid speciation may have occurred. Hook-nose and Maiasaura seem to fit into this scheme differently. With hook-nose we can see good evidence of rapid radiation of descendant species as the coastal plain opened up. All hook-nose's evolutionary grandchildren are out there with a whole spectrum of elaborated nasal arches. They'd been evolving up a storm. But Maiasaura is from the same generation as these grandchildren, and she is very little different from her ancestors. It seems that she proceeded quietly, evolving gradually, refining certain important characteristics such as teeth.

The difference, I think, lay in geography. I suspect that Maiasaura'?, ancestor appeared at the same time that hook-nose emerged, both in the mountains. When the new coastal plain opened up, however, the two ancestors took different directions. While hooknose colonized the new territory and spawned opportunistic descendants, the maiasaur ancestor stayed near the mountains on the upper part of the coastal plain. This was not as lush a territory. It may have had fewer niches, with fewer opportunities for radiation, and instead of providing the staging ground for numerous new species to emerge, it provided a testing ground to improve the ones that stayed there.

This is the kind of idea that one can test in paleontology, but not on the Peebles ranch. This land contains sediments from only the upper middle part of the Two Medicine formation. I needed to go to the bottom and look for a contemporary of hook-nose that could have been the ancestor of Maiasaura. And I needed to look a little bit higher in the formation for creatures intermediate between hook-nose and his presumed descendants. Such finds would provide examples of a transitional species, showing the process of evolution, something that had not been done, in this kind of detail, with dinosaurs.

This was one of the ideas generated by the finds on the Peebles ranch, and eventually I followed it up. But that was years in the future. My ideas about Maiasaura'?, evolution did not even begin to take shape until we had been digging up fossils from the Peebles ranch for several years. Where I left off in the story of the dig itself was the summer of 1978. Bob and I had found one nest and a skull. We had another six years of discoveries to go.

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